Campaign financing, reconfigured by the Supreme Court's 2010 Citizens United decision, allows individuals and corporations to contribute unlimited amounts of money to super PACs so long as the entities do not coordinate with candidates. Adelson and his family donated a combined $21 million to Gingrich's super PAC, helping Gingrich's resurgent campaign endure an onslaught of attack ads bankrolled by Romney and his allies.

But as Gingrich's campaign began to flag, Adelson signaled that he would be open to backing Romney. He has now put his money where his mouth is, sending $10 million to the presumptive Republican nominee's super PAC.

Adelson also spent $250,000 to help Wisconsin Governor Scott Walker stave off a recall effort, joining a cohort of out-of-state donors who made the Wisconsin recall the most expensive in the state's history.